Traditionally, VCSEL dies are vertically mounted to a printed circuit board, or PCB, with light emitting from the same surface as the electrical contacts. The PCB is usually made of FR4 or ceramic. As shown in the prior art of FIG. 1, a TO assembly can 12 has wire bonds 16 used in electrically connecting the VCSEL die 14. Wire bonds 16 are more susceptible to damage than solder bumps, and are generally avoided if possible. In addition, wire bonding is inconsistent in terms of variance in electrical properties. As the wire lengths tend to vary, variance exists in resistance, inductance, or capacitance along the lines.
As shown in FIG. 1, the TO can's base comprises a header 20 and a conductive spacer 18. A metallic structure 22, referred to as a can, provides a hermetic seal for a VCSEL laser array 14. Optical signals 26 exit the TO can 12 through a lens 24, and may be appropriately coupled into a waveguide (not shown). Lensing mechanisms are often needed to couple light as desired into a waveguide or optical fiber. For example, a VCSEL laser die contains electrical contacts on the same surface of light emission, and wire bonding to that surface will increase the minimum distance from the active surface of the laser to the optical fiber. As a result, the signal may require lensing to gather diverging light.
Additionally, multiple optical devices on the same substrate, such as VCSEL's on a PCB, can have a number of optical losses and/or interferences associated with them, one of which is cross talk. Light emitting from an optical port diverges from a normal transmission axis and could interfere with light from other optical ports. By increasing the optical device density on a substrate, the need to accurately couple light from an optical port into its respective waveguide or fiber increases. If the distance from a coupling device to the optical port increases, less of the divergent light will be collected. Given the feasibility limitations in changing optical fiber diameter, complicated lensing mechanisms are needed to appropriately couple light as desired.
FIG. 2 shows another prior art optical converter assembly 30. In this method of transmitting optical signals through an optical fiber 38, light strikes an end surface 40 of the optical fiber 38 and reflects into the optical fiber 38, along the longitudinal axis. As shown, wire bonds 42 are used to electrically connect the optical transmitting device 34 to a first substrate 32. Because of the position of the wire bonds 42, the position of the optical fiber 38 is limited to above the wire bonds 42.
In addition, this method of coupling optical signals requires precision optical alignment structures 36. The assembly does not allow the optical fiber 38 to be placed relatively close to the optical transmitting device 34, possibly reducing optical integrity. Complex alignment structures are thus required to gather light as desired.
A method of attaching a VCSEL die using metal to metal contacts such as solder bumps or stud bumps can make closer connections that are more consistent in electrical variance and offer greater structural stability than wire bonds. This method of attaching is commonly referred to as flip chipping. Wire bonding will add to the overall height in the package more so than flip chipping, as shown in FIGS. 1 and 2. As a result of flip chipping, more light could be gathered from an optical port into a waveguide or optical fiber. By flip chipping an optical array to a substrate, optical fibers can be placed closer to the active surface of the laser, possibly eliminating the need for lensing structures, while maintaining signal integrity. In addition, closer optical fibers to the optical array surface can reduce optical cross-talk caused by the scattering of light.
In moving a wave guide or optical fiber closer to an optical port, the total amount of light gathered can increase. As it is desirable to uniformly collect light over the optical source's total emission field, it is not necessarily advantageous to gather as much light as possible. Capturing too much light could cause a few problems, one of which is eye safety. As a laser can cause permanent damage to the human eye, it is imperative to ensure that a laser's output does not come in contact with a human eye in a hazardous manner. To do this, the net amount of light output could be decreased or absorbed in order to promote an eye safe device. That is, enough light output could be decreased or absorbed to maintain signal integrity but promote eye safety.
Another possible consequence in gathering too much light involves the inability of a receiving optical device to process the light energy. A photo detection device may provide an electrical output that is proportional to the amount of light energy received from a transmitting device. If the input signal to a photodetector contains too much light energy, the photodetector could become saturated. That is, the linear proportionality between the incoming light energy and the outgoing electrical signal could diminish, and the photodetector may not respond accordingly to too much light energy. Additionally, if the photodetector has not already saturated, the signal processor receiving the electrical signal from the photodetector could become saturated. That is, the signal processor's limits will have been reached because the value of the input electrical signal could be too high. Because of these two consequences in gathering too much light energy, it is necessary to appropriately reduce the optical energy received by a photo detection device.
The present invention provides a novel way to couple light from an optical device, into a waveguide, and subsequently into an optical fiber. The invention offers ease in manufacturing and does not require the use of precision alignment devices typically needed for the manufacturing of optical devices. It may simultaneously function as a spatial field homogenizer and an optical attenuator for an optical transmitting device. It may also allow a higher optical device density on a common substrate while not adversely affecting the signal integrity. In addition, the invention may promote eye safety while maintaining signal integrity.